|Publication number||US7447268 B2|
|Application number||US 10/815,097|
|Publication date||Nov 4, 2008|
|Filing date||Mar 31, 2004|
|Priority date||Mar 31, 2004|
|Also published as||US20050220199, WO2005099211A1|
|Publication number||10815097, 815097, US 7447268 B2, US 7447268B2, US-B2-7447268, US7447268 B2, US7447268B2|
|Inventors||John S. Sadowsky, Shmuel Levy, Sumeet Sandhu, Keith Holt|
|Original Assignee||Intel Corporation|
|Export Citation||BiBTeX, EndNote, RefMan|
|Patent Citations (34), Non-Patent Citations (1), Referenced by (19), Classifications (10), Legal Events (3)|
|External Links: USPTO, USPTO Assignment, Espacenet|
The invention relates generally to wireless communications and, more particularly, to multicarrier wireless communications.
One of the problems that must often be overcome in a wireless communication system to ensure reliable, high quality communication is multipath fading. In a wireless channel, it is often the case that a transmitted signal will reach a receiver by more than one path. That is, the signal may reach the receiver by a direct path from the transmitter and also by one or more other paths that involve signal reflection from objects or structures in the surrounding environment. Because the lengths of the different paths will typically be different, the corresponding signal components received at the receiver will usually have different phases. Sometimes the phasing of the received signal components will result in partial or full cancellation of the signal at the receiver. This loss of signal at the receiver is known as multipath fading. In some situations, the fading will be frequency selective. That is, the fading will be more pronounced at some frequencies than at others. In other situations, however, the fading may occur in a relatively uniform fashion over a larger bandwidth. This may be referred to as non-frequency selective or “flat” multipath fading. Many techniques exist for effectively dealing with frequency-selective multi-path fading. However, there is a need for methods and structures that are capable of addressing flat multipath fading, particularly in systems that utilize multicarrier communication techniques.
In the following detailed description, reference is made to the accompanying drawings that show, by way of illustration, specific embodiments in which the invention may be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to practice the invention. It is to be understood that the various embodiments of the invention, although different, are not necessarily mutually exclusive. For example, a particular feature, structure, or characteristic described herein in connection with one embodiment may be implemented within other embodiments without departing from the spirit and scope of the invention. In addition, it is to be understood that the location or arrangement of individual elements within each disclosed embodiment may be modified without departing from the spirit and scope of the invention. The following detailed description is, therefore, not to be taken in a limiting sense, and the scope of the present invention is defined only by the appended claims, appropriately interpreted, along with the full range of equivalents to which the claims are entitled. In the drawings, like numerals refer to the same or similar functionality throughout the several views.
The phase shift unit 12 applies a subcarrier dependent phase shift to each of the modulation symbols Xn input thereto. For example, with reference to
The transmitter arrangement 10 is intended to be used in a multiple transmit antenna system. That is, two or more of such arrangements 10, each having its own antenna, are to be implemented within a transmitter. The subcarrier dependant phase shift sequences used for the different antennas in the multiple antenna system will typically be different. By providing these subcarrier dependent phase shifts to the modulation symbols, a wireless channel that is displaying flat multipath fading characteristics may be converted into a frequency selective fading channel that can then be dealt with in a known manner.
As described above, the phase shift unit 12 applies a subcarrier dependent phase shift to each of the modulation data symbols input thereto. A variety of different approaches may be used for determining the phase shifts based on subcarrier. In one approach, for example, a phase term that is linearly related to the corresponding subcarrier frequency may be used, as follows:
where fn is the subcarrier frequency of the nth subcarrier relative to the channel center frequency and α is a constant. Other methods for generating phase terms that are linearly related (or non-linearly related) to subcarrier frequency may alternatively be used.
In at least one embodiment, features of the invention are implemented in a multiple input multiple output (MIMO) based multicarrier system. In a MIMO based system, multiple transmit antennas are used to transmit into a wireless channel (multiple input) and multiple receive antennas are used to receive the signal from the wireless channel (multiple output). MIMO systems are characterized by their ability, in the presence of rich multipath, to significantly increase the peak throughout that would otherwise be achievable in a single input, single output (SISO) system, without increasing the frequency bandwidth.
By inserting subcarrier dependent phase factors into the modulation data symbols associated with at least one of the two transmit antennas 38, 48, a wireless channel suffering from flat fading can be converted into a frequency selective channel that can be dealt with in a known manner. In an alternative arrangement, a different subcarrier-dependent phase sequence may be applied for each of the two transmit antennas 38, 48. Although shown with two transmit antennas 38, 48 in the illustrated embodiment, it should be appreciated that MIMO based transmitter arrangements having more than 2 transmit antennas may alternatively be used. In such an arrangement, at least one of the transmit antennas should apply subcarrier dependent phase shifts to the corresponding modulation symbols, as described above. In one possible implementation, subcarrier dependent phase shifts are applied for more than one transmit antenna in the multi-antenna arrangement, with a different subcarrier dependent phase sequence being used for each antenna.
In at least one MIMO based embodiment of the invention, a set of orthogonal or quasi-orthogonal spatial weights are applied to modulation symbols across groups of tones that are within a coherence bandwidth (B) of one another. Coherence bandwidth refers to a range of frequencies within which frequency components tend to all fade in a similar or correlated fashion in a system. In one possible phase shift approach, for example, a phase shift of 360/B (degrees) is applied from tone to tone across each group of B tones associated with at least one of the transmit antennas in a multi-antenna arrangement.
In another approach for use in a MIMO based system, the same signal may be transmitted on different antennas by successively delaying the signal on each antenna. That is, the signal transmitted from the Mth transmit antenna is cyclically delayed by (M−1)D time samples with respect to the first antenna, where D is the cyclic delay. This technique is capable of making the transmitted signal appear omnidirectional. The cyclic delay may also be adapted to the channel delay spread. For example, a smaller cyclic delay may be used with lower delay spreads and a larger cyclic delay may be used with higher delay spreads. In general, delay diversity can be proprietary and does not require standardization.
One of the problems that arises when using multiple transmit antennas that each transmit the same signal (such as in a MIMO based system) is that a directional antenna pattern often results. In at least one embodiment of the present invention, subcarrier dependent phases are selected for subcarriers associated with at least one transmit antenna in a multiple transmit antenna arrangement in a manner that results in an omni-directional aggregate radiation pattern over all of the subcarriers. Although the radiation pattern of each individual subcarrier cannot typically be made omni-directional using this technique, the aggregate pattern over all of the subcarriers can be made to be substantially omni-directional in an open loop transmit diversity system. As before, subcarrier dependent phases may be applied to a single antenna or to multiple antennas (using different subcarrier dependent phase sequences) in the multi-antenna arrangement.
In at least one embodiment of the present invention, the steering unit 88 is used to provide subcarrier dependent phase terms to the data symbols that will be output to at least one of the antenna paths 104, 106, 108. The subcarrier dependent phase terms may be selected as described previously. In at least one approach, an additional matrix multiplication may be performed within the steering unit 88 to provide the phase terms to the desired modulation symbols. For example, if the steering matrix to be used in a particular implementation is represented as V*, then the phase terms maybe introduced by modifying the steering matrix as follows:
The linear phase term as just described is applied after the fixed steering matrix V*. In this case, the phase term
is applied to the i'th antenna. In a MIMO system, it is possible to have more transmit antennas than spatial streams. The number of rows of V is the number of transmit antennas and the number of columns is the number of spatial streams. Now, in addition to applying the phasing per antenna, it is also possible to apply phases per spatial stream. In this case, we have the reverse order of V* and D:
and the dimension of square matrix D(ƒ;τ) is now the number of spatial streams (rather than the number of transmit antennas).
Although various embodiments have been described above in the context of MIMO based systems that typically have multiple receiver antennas, it should be appreciated that the above described principles may also be implemented in systems that use multiple transmit antennas and only a single receive antenna.
The inventive techniques and structures may be used in any of a wide variety of different wireless devices, components, and systems. For example, in various embodiments, features of the invention may be implemented within laptop, desktop, palmtop, and/or tablet computers having wireless networking functionality, personal digital assistants (PDAs) having wireless networking functionality, cellular telephones and other handheld wireless communicators, pagers, network interface cards (NICs) and other network interface structures, radio frequency integrated circuits, and/or other devices, systems, and components.
It should be appreciated that the individual blocks illustrated in the block diagrams herein may be functional in nature and do not necessarily correspond to discrete hardware elements. For example, in at least one implementation, two or more of the blocks within a block diagram (e.g.,
In the foregoing detailed description, various features of the invention are grouped together in one or more individual embodiments for the purpose of streamlining the disclosure. This method of disclosure is not to be interpreted as reflecting an intention that the claimed invention requires more features than are expressly recited in each claim. Rather, as the following claims reflect, inventive aspects may lie in less than all features of each disclosed embodiment.
Although the present invention has been described in conjunction with certain embodiments, it is to be understood that modifications and variations may be resorted to without departing from the spirit and scope of the invention as those skilled in the art readily understand. Such modifications and variations are considered to be within the purview and scope of the invention and the appended claims.
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|International Classification||H04L27/28, H04L27/26, H04L1/06, H04J99/00|
|Cooperative Classification||H04L27/2634, H04L1/06, H04L27/2602|
|European Classification||H04L27/26M3, H04L1/06|
|Aug 30, 2004||AS||Assignment|
Owner name: INTEL CORPORATION, CALIFORNIA
Free format text: ASSIGNMENT OF ASSIGNORS INTEREST;ASSIGNORS:SADOWSKY, JOHN S.;LEVY, SHMUEL;SANDHU, SUMEET;AND OTHERS;REEL/FRAME:015089/0985;SIGNING DATES FROM 20040728 TO 20040815
|Apr 25, 2012||FPAY||Fee payment|
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